Simply unlock and complete your choice of any 4 of the 6 Showdown Races.
The Showdowns you can choose from are:
1. Hammerhead Racing Showdown at the end of the Touring category
2. Vulpini Racing Showdown at the end of the Stock category
3. Disruptor Showdown at the end of the Tuner category
4. Euro Rand Showdown at the end of the GT category
5. Fernando Alonso Showdown at the end of the FA Racing category
6. Aurora Motorsport Showdown at the end of the Invitational category
Please note that not all races in each category need to be completed to unlock their respective Showdown and that you do not have to place first in all the races to be considered complete (some require top 1/2, top 3, etc..)